INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 123 



be the causes of colors among animals? What are the uses of colors? 

 What is albinism? Where have you seen instances of it? See Fig. 60. 



FIG. 60. 



FIG. 60. Albino Opossum (Didelphys virginiana). Photo by Folsom. 



Questions on the figure. What is albinism? In what structures is it 

 manifest? Among what groups of animals can you find that it occurs? 

 To what is color in hair due? What natural conditions tend to produce 

 color in organisms? What is the chief value of color as an adaptation, 

 individual and social ? 



161. Parasitism. Of a nature which combines the quali- 

 tes of commensals and of the preying animals is the association 

 known as parasitism. It is an association of individuals of 

 different species in which one member (the parasite) gets all 

 the benefits, and the other (the host) suffers the loss. It is 

 a case where one species preys on another, but in which it is 

 to the advantage of the parasite, especially if a permanent one, 

 as well as of the host that the life of the latter shall not be 

 suddenly destroyed. It will be readily seen that the parasite 

 increases the work to be done by the host, thus being a handi- 

 cap in the struggle for existence. This might easily bring 

 about the destruction of the species which serves as host were 

 it not for the fact that nearly or quite all animals support 

 various parasites. Parasites are of two classes, external, as 

 the fleas, lice, and the like ; or internal, as most of the parasitic 

 worms. The fleas are transient parasites, as are many other 

 insects which are free in the adult stage but lay their eggs in 



